In present digital cellular mobile radio communication systems, such as the GSM and D-AMPS systems, a call-setup allocates a fixed gross bit rate channel for a subsequent fixed bit rate speech service. This implies that the net bit rate conveying the speech information and also the amount of added redundant bits which are used for channel error protection are fixed. A compromise has to be made between the quality of the speech service, the gross bit rate and the degree of channel error protection:
On one hand a maximum speech quality requires a high net bit rate and a high gross bit rate.
On the other hand the system resources are limited and the system should be able to accommodate a very large number of users at any given time.
Since the total maximum gross bit rate that simultaneously can be transmitted by the system is limited, the system capacity is limited to a fixed maximum number of users that may simultaneously use the system within one cell. In order to accommodate a maximum number of users of the speech service at any given time, the net bit rate, the degree of channel error protection and thus the channel gross bit rate are set to certain minimum values which still guarantee a certain minimum degree of speech quality under various radio conditions.
The fixed setting of gross bit rate, degree of channel error protection and net bit rate and service causes the following problems:
The amount of channel error protection is fixed to such value that a certain level of speech quality is maintained in situations of a low C/I level, e.g. at cell borders. In some situations, however, a higher degree of channel error protection (with the cost of a lower net bit rate) could give a higher speech quality (more robust transmission). On the other hand, in situations of a high C/I level the degree of channel error protection is unnecessarily high, a considerable amount of protection bits being wasted. In such a case the speech service quality could be higher at a higher net bit rate (more accurate speech encoding) and a lower degree of channel error protection.
The fixed gross bit rate causes a hard limit of possible simultaneous users. Thus, in situations of high system load the risk of overload is high. An overload may result in failures of connection establishments and lost connections. In the opposite situation of low system load there is a lot of unused system capacity which is in principle free to be used for transmission at a higher gross bit rate and therefore a higher service quality.
The current inflexible use of a fixed speech service at a fixed gross bit rate and a fixed degree of channel error protection makes it impossible for an operator to offer selected services that could depend on the current situation, e.g. network load, time of day and date, location, etc. Moreover, it is also impossible for the user to select a more user-suited service.
In the current inflexible system speech decoding of the bit stream received from the mobile station is performed at the network side. This ignores the kind and the capabilities of the other terminal. Such a system may lead to quality degrading due to speech codec tandem configurations, e.g. in the case of a mobile station-mobile station connection.